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NDT procedures for hot tapping

When Industrial Member Comex UK Ltd needed to develop NDT procedures for hot tap connections, TWI was asked to take on the job. The work formed part of a larger Comex development programme which has the support of a group of oil companies and the UK Department of Energy. Its objective was to develop suitable equipment and procedures for hot tapping subsea oil or gas pipelines under hyperbaric conditions. The company plans to offer a hot tapping service to offshore operators when the project has ended.

TWI's Structural Integrity Department was first involved in 1988 when it reviewed suitable NDT methods, equipment and operator requirements. It recommended use of magnetic particle and ultrasonic inspection, not only because of the problems posed by the branch geometry to be inspected, but also because of the limitations of external access. With these restrictions special inspection procedures are necessary to ensure joint integrity.

In the second phase of the NDT work, the TWI team used a defect specimen made by Comex UK to develop inspection procedures. It consisted of a 914mm OD x 25mm wall main pipe with a 508mm OD x 25mm wall branch welded on. The specimen also incorporated reinforcement plate welds and examples of buttering. All these weld details contained deliberate defects, exact details of which were not known during the test programme. As none of these defects were surface breaking, the emphasis in technique development was on manual ultrasonic testing. Once the results of this test programme had been analysed, a set of inspection procedures was formulated and used to inspect a hot tap welded testpiece in air to determine its practicality.

The final phase of the complete programme involved preparatory work for a shallow water trial at the British Gas Blyth Dock facility. NDT development work at this stage consisted of selecting equipment for use under hyperbaric conditions, defining defect acceptance criteria for ultrasonic testing, and further modifications to procedures to include these changes.

For more information, please email contactus@twi.co.uk.

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